Automotive evaporative emissions control systems are known in which fuel vapor from a fuel supply are trapped in a charcoal canister so as to not be released to the atmosphere. Fuel vapors can be rapidly generated under severe automotive vehicle operating conditions. The canister should be maintained in a condition providing for capture of even rapidly generated fuel vapors by periodically purging the vapors trapped therein. Canister purge may be provided by applying engine intake manifold vacuum to the canister to draw the trapped vapor out of the canister and into the engine intake manifold where it is combined with engine intake air. The purged vapor has a significant effect on engine air/fuel ratio, and can perturb air/fuel ratio away from a desirable ratio, reducing engine performance and increasing engine emissions. Accordingly, it has been proposed to control the purge rate by positioning a purge valve in the vapor conduit between the engine intake manifold and the canister and controlling the valve position. It has further been proposed to reduce the purge rate when the purge vapor may be influencing negatively engine air/fuel ratio. For example, the engine air/fuel ratio may be determined as a function of the oxygen content in engine exhaust gas. If the determined air/fuel ratio deviates appreciably away from a desired air/fuel ratio, purge rate may be adjusted. Accordingly, maintenance of the canister can be compromised by other engine control operations. Further, the correction in purge rate is only made reactively, after the air/fuel ratio deviation and the corresponding emissions and performance penalties have been incurred. Under conditions in which purge rate, engine intake air rate, or engine fueling rate change rapidly, repeated deviations in air/fuel ratio may result from such reactive engine air/fuel ratio control.
The concentration and flow of purge vapor can vary significantly. A single purge valve position command can be associated with a wide range of actual delivered purge fuel vapor mass to the engine cylinders. To provide for precision control of the purge mass actually reaching the engine, it has been proposed to estimate the purge vapor flow passing between the canister and the engine based on a measurement of vapor concentration, and to adjust purge valve position in response thereto. However, vapor flow rate is dependent on vapor concentration and can vary significantly depending on variation in flow restrictiveness and during engine transient maneuvers. Estimates of purge flow rate fail to account for such factors as varying restrictiveness of the purge line, and certain transient conditions. Further, the mass of fuel vapor reaching the engine, such as the engine intake manifold, may not correspond exactly to the mass of fuel vapor actually reaching the engine cylinders due, for example, to vapor transport dynamics in the engine. As a result, the mass of purge vapor entering engine cylinders can only roughly be approximated. If aggressive purge control is desired, an engine air/fuel ratio control penalty must then be paid. As described, closed-loop engine air/fuel ratio control relying on exhaust gas oxygen sensor information may relieve this penalty under certain operating conditions. But if precise engine air/fuel ratio control is required under all engine operating conditions, maintenance of the canister may be compromised by reducing purge rate below that required to provide a canister with sufficient reserve capacity. Such compromise can increase system susceptibility to vapor release to the atmosphere. It has further been proposed to vary purge rate as a function of engine intake mass airflow rate, which again can compromise canister maintenance, for example, by reducing purge rate below a desired rate in response to changes in engine intake mass airflow rate. Accordingly, current purge control proposals suffer shortcomings in purge control accuracy, canister maintenance, and engine air/fuel ratio control accuracy.
It would therefore be desirable to determine the precise mass of purge fuel vapor entering individual engine cylinders, to correct the purge command in response thereto, and to provide for cylinder fuel injection in response to the determined vapor mass entering the cylinders so that accurate engine air/fuel ratio control under all operating conditions need not be compromised by the purge control operations necessary to maintain a canister capable of trapping vapors produced under even severe operating conditions.